A switching transistor typically operated by a pulse-width modulator is connected between a direct-current voltage source and an inductive load such as a motor. A voltage-limiting element such as a zener diode is normally also connected to the source and to the switching transistor to operate same when the voltage exceeds a predetermined limit.
In such a switching transistor the breakdown voltage is normally such that it is not exceeded under normal conditions by the applied source voltage plus the switching voltage and the line and stray inductances. It is impractical to use a transistor whose breakdown voltage is so high that it exceeds the highest overvoltage that may be encountered. For this reason it is standard to use a protective circuit by means of which the switching transistor is protected against overvoltages that exceed its breakdown voltage.
Thus it is known to connect a voltage-limiting element such as a varistor or zener diode across the switching transistor. The avalanche or zener voltage of such a device is selected below the breakdown voltage of the switching transistor so that when a potentially dangerous overvoltage occurs, this transistor is shunted out. It is therefore necessary that such a voltage-limiting element be large enough that it can pass relatively massive amounts of energy and can dissipate the heat generated by its own losses.
Another solution uses such a voltage-limiting device to actually switch the switching transistor. This makes it possible to use a relatively small zener, avalanche diode, or varistor since it merely serves to feed a blocking voltage to a control input of the switching transistor which itself is then open-circuited. The switching transistor can normally dissipate larger losses than the voltage-limiting elements.
This latter solution has the disadvantage that the switching transistor has only limited capacity to dissipate losses. Thus overvoltages which last quite some time, as is the case with line-voltage sources, can eventually overheat and ruin the switching transistor.
In order to avoid such long-term loading and burnout of the switching transistor it has further been suggested to provide circuitry that monitors the source voltage and that feeds to the pulse-width modulator a blocking signal when an overvoltage is detected. This causes the switching transistor, under the control of the modulator, to open circuit and stay open as long as the overvoltage exists. This reduces the voltage on the switching transistor to the switched voltage. Such an arrangement does protect the switching transistor, but has the considerable disadvantage that it results in complete shutdown of the system, that is deenergization of the load. In other words the motor being controlled will stop when the supply voltage exceeds the threshold level, something that normally cannot be tolerated.